In recent years, from the social requirement of lower fuel consumption and less exhaust emission, an electric automobile and a hybrid automobile, each of which is mounted with an AC motor as a power source of a vehicle, is drawing attention. For example, in some of the hybrid automobiles, a DC power source made of a secondary battery or the like and an AC motor are connected to each other via an electric power conversion device constructed of an inverter and the like, and a DC voltage of the DC power source is converted into an AC voltage by the inverter to thereby drive the AC motor.
In the control device of the AC motor mounted in this kind of hybrid automobile and electric automobile, there has been known a technique of “one phase control” in which a current sensor for sensing a phase current is provided in one phase and in which current estimated values based on current sensed value of the one phase are fed back, whereby current flowing through the AC motor is controlled (for example, see patent document 1). According to the technique, the current sensor is provided only in the one phase, so that the number of current sensors is reduced and hence a construction near three phase output terminals of an inverter can be reduced in size and the control system of the AC motor can be reduced in cost.
According to a current feedback control based on the technique of the patent document 1, of three phase current command values acquired by inversely dq transforming d axis and q axis (hereinafter referred to as “dq axis”) current commands, current command values of two phases other than a sensor phase are treated as current estimated values. Then, a current estimated value of the sensor phase of one phase (for example, a W phase) and dq axis current estimated values acquired by dq transforming the current estimated values of other two phases (for example, a U phase and a V phase) are fed back to dq axis current commands id*, iq*.
In the meantime, a state is assumed in which an AC motor is stopped and in which a current command that is not zero is commanded. Depending on an electric angle and the phase of a current command vector, there can be a case in which the AC motor is stopped at a position in which the phase current of the sensor phase becomes zero. In this case, the current sensed value of the sensor phase is always sensed to be zero.
Here, it is assumed that a term of “stop” includes a low rotation state in which the number of revolutions is not more than a given number of revolutions. Further, at the time of saying that the current is “zero”, a term of “zero” includes not only a value of strict 0 [A] but also a value within the substantially same range as 0 [A] in consideration of the sensing error and the resolution of a device. Moreover, at the time of saying that the current is “not zero”, the phrase of “not zero” does not mean to exclude only a value of strict 0 [A] but means a value outside the substantially same range as 0 [A].
In this way, when a current sensed value of a sensor phase is always sensed to be zero, in the current feedback control based on the technique of the patent document 1, dq axis current estimated values, which are acquired by dq transforming a value of zero that is a current sensed value of the sensor phase and current estimated values of other two phases, correspond to dq axis current commands id*, iq*. Hence, the dq axis current commands id*, iq* result in being fed back to dq axis current commands id*, iq* as they are, so that dq axis current deviations Δid, Δiq become zero, which hence brings about the same state in which the feedback control is not performed.
For this reason, when the AC motor is stopped at a position in which the current sensed value of the sensor phase is always sensed to be zero according to the electric angle and the phase of the current command vector, in the current feedback control based on the technique of the patent document 1, the drive control of the AC motor is likely to be made unstable.    [Patent Document 1] JP-A No. 2008-86139 (corresponding to US 2008/0079385-A1)